For best results, use high quality, high resolution
images. Remember that a wallpaper must be 1024x768 pixels in order
to be accepted onto my site, and an image that you use on the
wallpaper would have to cover a good portion of it. So, you probably have to
start with an image about 700 pixels high (wallpapers generally have
more horizontal than vertical whitespace). Electronically enlarging an image is
not a good idea---the quality degradation is very noticable. Avoid screen
captures from video files that are not DVD quality.

You can purchase and scan anime artbooks to get the highest
quality images. This is admittedly a somewhat expensive route, but
you get the best selection of images this way, and can scan them at an
appropriately high resolution for wallpaper making. When scanning,
make sure that the artbook is held absolutely flat on the scanner.

Some good search engines for images are Media Miner and Google Image Search. Both of these search
engines show thumbnails of the pictures, as well as their dimensions.
Also on my links page are some great links to
some picture sites.

Clean artifacts out of the image. Artifacts are when the
image has little uneven squares of color here and there. You can
remove artifacts from an image by carefully skilled blurring and
smudging and/or zooming in on the image and manually repainting some
areas. If you can start with a clean image in the first place though,
you can avoid this step altogether.

Use colors that go well with the image that you're
using. The whole wallpaper should fit well together.

Test how the wallpaper looks on your desktop.
Take into account that:

most desktops have a taskbar covering the bottom
(this is a good place to put your signature; it will identify the
wallpaper as your work, without disturbing the look of the
wallpaper)

most desktops have several columns of icons,
generally on the left side of the screen; the wallpaper should have
some whitespace set aside for this, since icons look bad on a busy
background

When saving your image, do not overly compress it. Good 1024x768 wallpapers
are generally 100kb, with some as large as 300kb. Successive saving and
loading of JPGs causes quality degradation, so when you are saving an
image that you intend to work more on later, use a non-lossy format (such
as PNG or PSD). When you finally submit your wallpaper to us, you should
send it either in JPG format (with quality factor no lower than 95%),
or PNG format. If you are in doubt as to how much you can simply compress
an image, pick a higher quality factor; I can recompress the image if
necessary. Do not save it as a GIF; GIFs only support palettes
of up to 256 colors, which is insufficient for good quality.

Be prepared to spend a good deal of time making a
wallpaper. Experienced wallpaper makers may be able to create a
great wallpaper in under 20 minutes, but it takes work either way. Do
not expect to create a good wallpaper just by taking an image and
applying a cloud filter background with a lens flare in
Photoshop. Speaking of lens flares, use them sparingly; a plain old
Photoshop lens flare can be pretty, but a common mistake is to use too
many lens flares, which gets cheesy.

Finally, remember that there can be beauty in simplicity. A
wallpaper doesn't have to be very "busy" in order to be beautiful; in
fact, a wallpaper should not be too busy, since it is meant to be used
as a desktop background which should add atmosphere to the
desktop around it, rather than draw attention to itself (which can get
distracting to someone who is trying to work on the computer!).